Good luck. Keep us posted on your decision.

One place could be the hatch. If it were left open, that is an obvious cause.  I suspect that is not the cause. The next thing is if the frame around the hatch were leaking. That would likely lead to some indication visible on the side wall around the hatch. Take a good look there. If nothing suspicious appears then it is time to move on.

Next thing would be to look at the front. Is there any evidence of separation between the front and the side wall? Is there any evidence of separation at the bottom of the front wall. Those places could be a cause. If you do not see anything there, then it is time to look at other causes. How is the floor in the rest of the trailer? Is this damage isolated to the storage compartment? There are water lines that cross over from the other side of the trailer to supply the bathroom and toilet. It is possible that the trailer was not properly winterized or something else happened to cause one of the pipes to break. If you remove the vent grill near or under the door of the bathroom (going from memory here), you may see if the water came from the bathroom or shower. It is also possible that the shower drain was leaking, which would have allowed water to seep into that visible area.

However, I can't be sure as I am only speculating as to the possible cause. I'm sorry this happened to you. Unfortunately, this being an estate sale, it is going to be hard to return it for a refund as those are generally sold as final with no returns. Can it be salvaged? I can't say. You need much better advice on that then I am capable or qualified to give. I hope you will find a workable solution to this problem.